The present invention relates to an apparatus and method for checking bank notes for their state of use, in particular for dirt and stains which can impair the service life of the bank notes.
For checking bank notes for dirt and stains, one usually illuminates the bank notes by means of at least one light source and evaluates the diffusely reflected light by means of suitable optical sensors.
However the problem arises that particularly places on the bank note which contain security features like watermarks are evaluated as stains or soiled places.
Further problems result from the fact that when a bank note is judged for its degree of soiling great effort is necessary for evaluating the signals of the sensors used since soiling or stains are felt to be especially disturbing in certain areas of the bank note, e.g. in the area of a portrait. The evaluation effort must be adapted for the whole bank note according to the critical areas and is thus especially great.
The problem of the present invention is therefore to state an apparatus and method for checking bank notes for their state of use, in particular dirt and stains which can impair the service value of the bank notes, which allow accurate judgment of the bank notes. The effort for judging the degree of soiling of the bank notes should advantageously be reduced.
The invention starts out from the consideration that two identical illumination and sensor units are disposed on both sides of a transport path for bank notes to be checked. The illumination and sensor units of both sides are disposed in such a way as to be aligned with a given place. Illumination at the same time and of the same kind from both sides avoids misjudgments in areas, e.g. in the area of the watermark. Additionally it is possible to judge the front and back of bank notes to be checked simultaneously.
One advantageously reduces the effort for judging the degree of soiling of bank notes by defining areas for the bank notes to be investigated where judgment is performed with greater effort, e.g. higher resolution, than in other areas. It has proved to be especially advantageous to fix the areas of higher resolution in accordance with the currency and/or denomination for the bank notes to be investigated.
Further advantages of the present invention can be found in the dependent claims and the following description of embodiments with reference to figures. The figures show only the components relevant for understanding the present invention. Similar components of the figures have the same reference signs.